imaginary number

A number whose square is negative. Every imaginary number can be written
in the form ib where b is a real
number and i is the imaginary unit
with the property that i 2 = -1. For example, √-16
is an imaginary number, since √-16 = √-1.√16 can be written
as 4i.

Imaginary numbers are complex numbers
in which the real part is zero. The term "imaginary" is unfortunate because
it suggests something that has less reality than a "real" number, which
isn't the case.